In general, the main components of existing superconducting magnetic systems are a cryostat, a system of cooling, a sectionalized superconducting coil and an energy output device providing for protection of the magnetic system in the event that normal zones should arise within the superconductor of the coil sections. The shape, dimensions and mutual arrangement of the coil sections depend on the purpose of the magnetic system. As magnetic systems employing superconductors are rather expensive installations, it is essential to prevent at interruptions in superconductivity occuring at the appearance of normal zones within the superconductor of the coil sections.
In the early stages of development of superconducting magnetic systems when the principles of stabilization of the superconductor, the technology of its manufacture and the principles underlying the design of the cryostat and the system of cooling were still rudimental, the main efforts of designers were directed to preventing the occurence of fluctuating normal zones (i.e., interruptions in superconductivity).
Thus, the layers of the superconducting coil of some superconducting magnetic systems (see, for instance, U.S. Pat. No. 3,363,207, 1968,) are kept at a certain distance from one another by supporting elements consisting of shields made of conducting material having a low electrical resistance. The shields form short-circuited loops, and both surfaces of each shield are covered with a layer of porous insulating material. The shields are provided with ducts for the flow of the cooling agent.
Such an embodiment of the superconducting magnetic system improves the stability of its operation by the introduction of electromagnetic stability (due to the short-circuited loops) and improvement of cryostatic stabilization (as the ducts increase the area of the cooling surface).
To improve the stability of operation by eliminating the sharp changes in magnetic flux, some superconducting magnetic systems [(see, for example, French Pat. No. 2,027,395, 1970,] have iron inserts producing intense magnetization arranged at sections where the components of the magnetic flux are at right angles to the superconducting surface of the strips forming the coil.
In other superconducting magnetic systems [(see, for instance, French Pat. No. 2026570, 1970,] comprising a cryostat, system of cooling and superconducting coil, the coil is cooled by forced circulation of the cooling agent, thus improving the cryostatic stabilization of the coil superconductor by raising the heat-exchange coefficient.
Known in the art are also superconducting magnetic systems employing all kinds of band elements to prevent degradation of superconductivity and lessen the probability of appearance of normal zones.
Therefore, in the course of development of the design features of superconducting magnetic systems, the appearance of normal zones within the superconductor was treated as an irreversible phenomenon making it necessary to stop the operation of the system. That stoppage is known to be accompanied by the withdrawal of the energy stored in the magnetic field of the system. In this case, the problem is that of removing the energy without causing any damage to the superconducting magnetic system. The problem is solved by resorting to various embodiments of the superconducting magnetic systems. A known superconducting magnetic system of this type (see, for example, French Pat. No. 2181218, 1974, Cl. is fitted with a protective device coil of superconducting material linked inductively with the superconducting coil of the magnetic system. At the occurence of a normal zone within the superconducting coil of the magnetic system, the coil of the protective device becomes energized and its magnetic field transfers the superconducting coil of the magnetic system into its normal (nonsuperconducting) state. This allows avoidance of an inadmissible temperature rise of the superconductor of the magnetic system at the point of initial formation of the normal zone.
Most versions of this type of solution of the problem are, however, aimed at passing the energy stored in the superconducting magnetic system over to the surrounding medium (T.apprxeq.300K). The energy may be passed over to the electric power supply mains, say, with the aid of an inverter, or dissipated as thermal energy in an external emergency shunt made, for instance, as a cooled resistor.
Known also is a superconducting magnetic system (see, for instance, Japanese Pat. No. 45-14999, 1970,) wherein the sections of the superconducting coil are connected in series with the source of supply under normal duty conditions of the system and in parallel with an external emergency shunt (with the source of supply disconnected) at the appearance of a normal zone within the superconductor of any of the coil sections. A much simplier solution of the problem is achieved in superconducting magnetic systems [(see, for example, FRG Pat. No. 1439487, 1973,], wherein the sections of the superconducting coil are shunted with normal metal of a resistivity higher than that of the stabilizing base layer of the superconductor. The magnetic system is also provided with an energy removal device comprising an external emergency shunt and disconnector of the supply source of the superconducting magnetic system. The energy removal device operates on receiving a signal from the normal zone detection unit. Such an arrangement ensures simultaneous energy removal and shunting of the superconductor in the process of removal, thus lowering the temperature of the superconductor.
None of the above versions provide for continuation of operation of the superconducting magnetic system after the occurence of a normal zone within the superconductor, though there is a great demand for such systems.
For example, it is essential to provide for continuous operation of the superconducting magnetic system and maintain its output at set level in cases where the magnetic system is a component of larger devices, say, devices wherein plasma is to be confined by the magnetic field.
Known is a superconducting magnetic system comprising a superconducting sectionalized coil, each section of which has a series-connected disconnector, a controlled shunt of superconducting material and a normal zone detection unit connected in parallel with the section and its disconnector and is coupled inductively with an energy removal device [see, for instance, USSR Inventor's Certificate No. 570283, 1975,].
The incorporation of a series-connected disconnector and parallel shunt of superconducting material in every section of the coil of the above-mentioned superconducting magnetic system allows to disconnect the section where the normal zone has appeared, maintain the electrical link between the remaining sections and the source of supply and localize the point of excessive temperature rise of the disconnected section.
The energy removal device of the system has a magnetic circuit ensuring a high coefficient of magnetic coupling of the energy removal device with any of the coil sections and, at the same time, allowing to control the shunts of superconducting material arranged within the gaps of the magnetic circuit branches. The employment of a magnetic circuit in the energy removal device limits, however, the field of application of such a superconducting magnetic system since the saturation of the magnetic circuit (occuring for most conventional ferromagnetic materials at a magnetic field density of 2T) worsens the magnetic coupling of the section with the energy removal device that--in addition to the magnetic circuit--has a superconducting protective coil, current leads and an external emergency shunt and disconnector. There are ferromagnetic materials saturating at cryogenic temperatures of 4T. They are, however, difficult to manufacture and, moreover, the density of the fluxes set up by superconducting magnetic systems is in most cases up to and above 6T. The superconducting protective coil is arranged--just as the sections of the superconducting coil of the magnetic system--on the above-mentioned magnetic circuit and connected by current-carrying leads in series with the external emergency shunt via a disconnector.
Under normal duty conditions of the superconducting magnetic system, the circuit of the protective coil is open.
At the occurence of a normal zone within any of the sections of the superconducting coil, the circuit of the section in question is opened and that of the protective coil is closed. The energy associated with the disconnected section is transferred with the aid of the protective coil to the energy removal device, dissipated in the external emergency shunt and--to a certain extent--in the disconnector of the given section. The dissipation of energy within the disconnector is accompanied by intensive evaporation of the cooling agent in the cryostat of the superconducting magnetic system. The same cooling agent serves for normal functioning of the sectionalized superconducting coil. The worsening of magnetic coupling is accompanied by excessive release of heat within the disconnector of the disconnected section and, consequently, is liable to cause an inadmissible rise in the pressure of the cooling agent, causing its ejection and may interrupt the operation of the superconducting magnetic system. The saturation of the magnetic circuit also leads to a drop in the field strength within the gaps of the magnetic circuit branches and untimely opening (superconductivity) of the shunts of superconducting material. At fluctuations of the magnetic field, this is liable to cause the transfer of a certain quantity of the energy over to the source of supply or--when using a superconducting element short-circuiting the superconducting sectionalized coil--to an unforeseen rise in the current of the superconducting sectionalized coil.
Besides, the current-carrying leads of the energy removal device may cause the transfer of the protective coil over to normal state, an event that also contributes to excessive dissipation of heat within the disconnector on disconnection of the section and, consequently, raising the pressure of the cooling agent and causing its ejection.